1. Field of the Invention
This invention relates to hot melt adhesives and to their use for bonding substrates together, especially substrates having low surface energies.
2. Summary of the Prior Art
Hot melt adhesives (also known as thermoplastic adhesives) are well known in the art and have valuable advantages in processing, storage and application. However, many of the known hot-melt adhesives will not bond well to materials having low critical surface energies, in particular fluorinated polymers, and/or are unstable (ie. they melt and flow and/or are chemically unstable) at the elevated temperatures (eg. above 100.degree. C.) at which the use of such materials is often particularly valuable. Although the bondability of such materials can in some cases be improved by a surface treatment such as chemical etching, the need to carry out such a surface treatment is a disadvantage. Other known hot melt adhesives, which will bond well to substrates of low critical surface energy, suffer from other disadvantages. For example, one material which has been used as a hot melt adhesive is fluorinated ethylene/propylene copolymer (often know as FEP). Unfortunately, however, in order to provide good bond strength, FEP must be heated to a temperature of over 300.degree. C., typically 315.degree.-370.degree. C., under a pressure which is typically 25-40 psi (1.75-2.8 kg/cm.sup. 2). It cannot, therefore, be used to bond substrates which melt or soften below such temperatures, eg. the fluorinated polymers. It has also been proposed to make hot melt adhesives based on ethylene/tetrafluoroethylene polymers, but these suffer from the same disadvantages as FEP copolymers. Hot melt adhesives based on mixtures of ethylene/vinyl acetate (EVA) copolymers and tackifiers are also known, but they do not bond well to materials which have low critical surface energies and/or which are non-polar, such as fluoropolymers; in addition their stability is poor at elevated temperatures eg. above 100.degree. C., especially above 130.degree. C., eg. at 150.degree. C., so that the bond strength is seriously reduced by exposure to such temperatures. The same is true of hot melt adhesives based on polyesters, which have the additional disadvantage, when used for bonding electrical insulation, that their electrical insulation properties are poor, particularly under conditions of high humidity. Hot melt adhesives based on polysiloxane block copolymers have also been proposed, but they are expensive and not readily available.
Solvent-based adhesives containing fluoroelastomers have also been proposed, but do not have the ease of processing, storage and application of hot-melt adhesives, and also require the presence of a curing agent and use of a curing cycle after application.